The present invention generally relates to tools and methods for removing or debonding orthodontic brackets which have been affixed to the teeth of a patient with an adhesive bonding material. More particularly, the tools and methods relate to debonding procedures intended to be carried out without breaking or fracturing brackets formed from hard, brittle materials, such as ceramic materials.
One of the major challenges associated with the use of ceramic brackets for orthodontic treatment is the removal of the brackets from the teeth of the patient. Metal brackets are generally removed by pinching the brackets in a generally mesial-distal or diagonal direction using a pair of pliers, such as so-called Weingart pliers. The pliers apply compressive forces and pinch the brackets generally in a mesial-distal direction, typically by placing the jaws of the pliers diagonally across the bracket and engaging tie wings at opposite corners of the bracket, for example, at the gingival/mesial and occlusal/distal corners. Pinching the metal bracket in this manner results in deformation of the ductile bracket body as well as the bonding pad used to attach the body to the tooth. This deformation causes a separation or fracture at the interface between the adhesive and the bonding pad or base and essentially peels the bracket away from the tooth as the adhesive material fractures or debonds.
Brittle orthodontic bracket materials, such as ceramic materials, are much more problematic during the bracket removal or debonding process. These bracket materials are extremely hard and non-ductile relative to the materials, such as stainless steel, typically used for metal brackets. Ceramic materials also have a low fracture toughness relative to steels and other metals, meaning that ceramic material is much more prone to fracture under an applied force rather than deforming under the applied force. An attempt to pinch the wings of a ceramic bracket in the manner described above for metal brackets generally results in fracture of the tie wings or other portions of the bracket due to point loading of the bracket material by the pliers at the contact points. For this reason and other reasons, such as heightened sensitivity to surface imperfections, various alternative debonding tools and methods have been proposed and used, however, none have been fully satisfactory.
As an example, plier-type tools having metal jaws with sharp, opposed edges have been used in the past with the sharp edges intended to be directed into the adhesive interface between the bracket base and the tooth of the patient. The jaws are then squeezed together in an effort to separate the base from the tooth by the application of forces directly to the adhesive interface. The problem with this tool and method is that it is often very difficult to accurately align and maintain the sharp edges of the jaws at the adhesive interface as the handles of the plier-type tool are squeezed to apply the requisite debonding forces. Another tool and method has involved gripping the bracket on its mesial and distal sides and using a torsional force to rotate the bracket about an axis perpendicular to the base of the bracket and shear the adhesive interface. The torsional force applied by this method can be very uncomfortable for the patient and often requires excessive force that could also be harmful to the tooth structure. Another past method and tool involves grasping occlusal and gingival sides of the bracket and pulling the bracket directly away from the tooth along an axis perpendicular to the bracket base. This method also requires an excessive amount of force making it difficult to separate the bracket from the tooth, and can result in damaging the tooth surface. Finally, a more recent method for debonding ceramic brackets involves compressing or pinching together mesial and distal sides of the bracket to fracture the bracket along a midline in a direction parallel to the tooth surface and perpendicular to the archwire slot.
To overcome various problems with debonding tools and methods of the past, and especially tools and methods for debonding hard, brittle brackets such as ceramic brackets, new tools and methods are needed for effectively debonding the bracket in one piece without breakage or fracture of the bracket and with minimal force, discomfort, and potential for harmful effects on the patient.
In one aspect, the present invention provides an orthodontic debonding tool for compressively gripping a nonmetallic, hard and brittle bracket, such as a ceramic bracket, which is bonded with an adhesive to a tooth of a patient. The tool includes a first jaw having an outer end with a bracket-engagement portion and a second jaw also having a outer end with a bracket-engagement portion. The second jaw is coupled to the first jaw to allow movement of at least one of the first and second jaws between an engaged position in which the bracket-bracket-engagement portions grip opposite sides of the bracket, preferably gingival and occlusal sides thereof, and a disengaged position for releasing the grip on the bracket. At least the bracket-engagement portions of the first and second jaws are formed from a material having a hardness sufficiently less than the hardness of the ceramic bracket so as to substantially reduce the probability of the bracket breaking or fracturing when the bracket-engagement portions of the jaws grip the bracket on opposite sides with a compressive force.
During debonding, a pivoting motion is applied to the first and second jaws about an axis generally lying in a plane parallel to the plane of the bracket base surface, i.e., the surface that abuts the tooth. In this manner, a tensile force is applied to one side of the bracket in a direction away from the tooth and the adhesive is fractured without breaking or fracturing the ceramic bracket. Alternatively, a compressive force directed toward the tooth, a smaller tensile force directed away from the tooth, or essentially no force may be exerted along the opposite side of the bracket to facilitate the pivoting motion. The xe2x80x9csidesxe2x80x9d of the bracket under the applied force(s) may be occlusal, gingival, mesial or distal sides, or a combination thereof such as a bracket corner.
The tool preferably includes first and second handles respectively connected to the first and second jaws with the handles adapted to be gripped by user and moved with respect to each other to apply the compressive force to the bracket. The first and second jaws may be integrally formed with the respective first and second handles, for example, by molding from a nonmetallic material such as plastic or a plastic composite. One advantage to such a construction is that the tool may be economically fabricated or formed as a disposable item and discarded after use on the brackets of a particular patient. In the preferred embodiment, the jaws and handles are formed from a glass-filled polycarbonate material, however, many other materials which are substantially softer than the conventional ceramic bracket materials may be used as well, including ductile metals, such as titanium, aluminum, brass, zinc or various metal alloys. Other materials useful for at least the engagement portions of the tool include, but are not limited to, polysulfones. Thermoplastic materials, thermoset materials, rubbers, other polymeric materials or even wood may be used for at least the bracket-engagement portions as well.
At least the bracket-engagement portions of the first and second jaws should have a modulus of elasticity less than about 15xc3x97106 psi. More preferably, the modulus of elasticity for those portions of the tool configured to contact the bracket should be less than about 5xc3x97106 psi. In terms of hardness, at least the bracket-engagement portions of the first and second jaws should have a Knoop microhardness of less than about 500 and, more preferably, less than about 300. This is substantially less than the Knoop microhardness of conventional ceramic bracket materials which typically have a Knoop microhardness of at least about 1,000-1,500 and more typically about 2,000. The orthodontic brackets suitable for use with the present invention have a modulus of elasticity higher than about 20xc3x97106 psi. The fracture toughness of the bracket material is preferably less than about 20 MPa{square root over (m)} and, more preferably, less than about 10 MPa{square root over (m)}.
Conventional metal brackets, on the other hand, have a fracture toughness of at least about 50 MPa{square root over (m)}. The significant difference between the modulus of elasticity and/or the hardness of the bracket-engagement portions and the bracket help ensure that point loading on the bracket does not occur as the jaws of the tool apply compressive force to grip the bracket and thereafter pivot to apply unequal debonding forces in the tensile direction. Instead, the bracket-engagement portions will slightly yield against the harder, more brittle bracket material and spread the applied forces across a wider area of the bracket. As the tensile force is applied, the adhesive interface will fracture or debond before the forces reach a level that could cause the bracket to fracture.
A force limiting feature may also be incorporated into the tool. For example, the bracket-engagement portion or portions may be designed to break or deform at a specified level of force which is lower than the force necessary to fracture the bracket or damage the tooth enamel. Optionally, the handles or other portions of the tool may be designed to contact each other to prevent further compressive movement of the jaws which could fracture the bracket or, during a pivoting motion, damage the tooth enamel.
In a more specific embodiment, the bracket-engagement portions of the first and second jaws include respective first and second nibs projecting toward each other in the engaged position from respective side walls of the engagement portions which extend transverse to the nibs. The nibs are configured to respectively engage recesses on opposite sides of the bracket, such as the recesses that are typically disposed between opposite tie wings of the bracket and the bracket base. In this embodiment, the side walls of the engagement portions preferably contact and apply a compressive, bracket-gripping force to the opposite tie wings of the bracket while the nibs are engaged in the recesses below the tie wings. The tool may then be pivoted in the manner described above to apply a tensile force to at least one side of the bracket in a direction away from the tooth and fracture the adhesive bond between the bracket base and the tooth without fracturing the bracket.
In another embodiment of the invention, a debonding tool comprises an elongate member having a longitudinal axis, a first or outer end adapted to be gripped by a user, and a second or inner end. A bracket-engagement member is disposed on the second end and extends generally transverse to the longitudinal axis for engaging a recess in a side wall of the bracket. A tooth-engagement member is located on the second end, preferably at the extremity of the second end outside of the bracket-engagement member, and lies generally along the longitudinal axis. The tooth-engagement member acts as a fulcrum when the bracket-engagement member is engaged with the recess and a prying motion is applied by pivoting the elongate member with respect to the bracket about an axis lying in a plane generally parallel to the plane of the bracket base surface, i.e., the surface facing the tooth. This pivoting action applies a tensile force along the recessed side wall of the bracket in a direction away from the tooth to fracture the adhesive between the bracket base and the tooth without fracturing the bracket. The bracket-engagement member and the tooth-engagement member preferably comprise respective projections extending from the second or inner end of the elongate member in directions which are generally perpendicular to each other. This tool may also be comprised of metallic or nonmetallic materials and may be formed from harder materials than the first embodiment since a compressive force is not being applied during its use, although at least the bracket-engagement member should be sufficiently softer than the bracket to avoid fracturing the bracket during debonding. Like the first embodiment, it is preferred that this tool be a disposable or single-use product, however, since it may be formed from harder materials, it may be even more appropriate as a reusable tool.
Generally, a method of debonding a nonmetallic bracket in accordance with the invention includes engaging at least a first side of the bracket, pivoting the bracket about an axis lying in a plane which is generally parallel to the plane of the bracket base surface to apply a tensile force to the first side of the bracket in a direction away from the tooth, and fracturing the adhesive bond between the bracket and the tooth under the tensile force applied to the first side to remove the bracket in a single piece. In one embodiment, the method involves engaging first and second opposite sides of the bracket with the tool and applying a compressive gripping force to the bracket prior to pivoting the tool.
In another general embodiment, a method of debonding a bracket having at least one tie wing on each of two opposite sides of the bracket base, and at least one recess disposed between the base and each tie wing, includes engaging the bracket with a portion of a tool positioned in at least one of the recesses, moving the tool portion about an axis lying in a plane generally parallel to a plane of the bracket base surface to apply a tensile force to the corresponding side of the base in a direction away from the tooth, and fracturing the adhesive between the bracket base and the tooth without fracturing or breaking the bracket. In the preferred embodiment of the methods of this invention, the bracket is formed from a hard, brittle, and fracture-prone material as compared to relatively ductile brackets formed from metal with a general example being a ceramic material.
Another debonding method in accordance with the invention includes engaging at least one tool engagement portion of a ceramic bracket with a debonding tool, moving the tool to apply unequal forces to opposite sides of the ceramic bracket with a greater tensile force being applied in a direction away from the tooth, and fracturing the adhesive bond between the base of the bracket and the tooth beginning at the side under the greater force and without breaking the ceramic bracket. The force on the opposite side, or pivot axis side, of the bracket may be essentially zero, or be a compressive force, or be a smaller tensile force directed away from the tooth.
Various additional features, objectives and advantages of the tools and methods summarized above will become more readily apparent to those of ordinary skill in the art, upon review of the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.